This invention relates to a mass spectroscope used in liquid chromatography, or a mass spectroscope for a liquid chromatograph (herein abbreviated into xe2x80x9cLC-MSxe2x80x9d).
As shown in FIG. 5, a representative example of currently available LC-MS may be described as consisting of a liquid chromatograph (LC) part 10, an interface part 20 and a mass chromatograph (MS) part 30, and a liquid sample which elutes from a column 11 in the LC part 10 in a time-wise separated manner is introduced into the interface part 20 and is sprayed into an atomization chamber 22 through a nozzle 21 to be ionized. The ions thus generated are passed into the MS part 30 through a solvent-removing tube 23 such as a heated capillary. The MS part 30 consists of a first intermediate chamber 31, a second intermediate chamber 32 and an analyzer chamber 33, the solvent-removing tube 23 and a skimmer 35 having an orifice with an extremely small diameter being provided respectively between the atomization chamber 22 and the first intermediate chamber 31 and between the first intermediate chamber 31 and the second intermediate chamber 32. The interior of the atomization chamber 22 is maintained approximately at an atmospheric pressure but the interior of the first intermediate chamber 31 is reduced to about 1 Torr by means of a rotary pump while the interior of the second intermediate chamber 32 and the analyzer chamber 33 is reduced by means of a turbo molecular pump respectively to about 10xe2x88x923-10xe2x88x924 Torr and about 10xe2x88x925-10xe2x88x926 Torr. In other words, it is so arranged that the degree of vacuity becomes progressively higher from the atomization chamber 22 to the analyzer chamber 33.
The ions which have passed through the solvent-removing tube 23 are caused to converge to the orifice of the skimmer 35 by means of deflector electrodes 34, pass through the skimmer 35 and are introduced into the second intermediate chamber 32. They are then transported into the analyzer chamber 33, being converged and accelerated by ion lenses 36, and only the target ions having a specified mass number (or the ratio between the mass m and its electric charge z) are allowed to pass through a quadrupole filter 37 disposed inside the analyzer chamber 33 and to reach a detector 38 which is adapted to output a current determined by the number of ions which have been received thereby.
The interface part 20 is for generating gas ions by atomizing the liquid sample through heating, a high-speed gas flow or a high electric field. The so-called atmospheric pressure chemical ionization (APCI) and electro-spray ionization (ESI) methods are most commonly used for this purpose. By the APCI method, a needle electrode is disposed in front of the forward end of the nozzle 21 and the ionization process is carried out by causing the drops of the sample liquid atomized by the heating at the nozzle 21 to undergo a chemical reaction with the carrier gas ions (buffer ions) generated by the corona discharge from the needle electrode. By the ESI method, a highly uneven electric field is generated by applying a high voltage of several kV to the tip of the nozzle 21. The liquid sample is separated according to the charge by this electric field and atomization takes place by the Coulomb attraction. The solvent in the liquid drops is evaporated by contacting the environmental air and gas ions are thus generated.
By either of these methods, the generated small liquid drops containing ions are introduced into the heated solvent-removing tube 23 and the evaporation of the solvent inside these liquid drops takes place while these liquid drops are transported into the first intermediate chamber 31. Since the spontaneous destruction of the liquid drops due to the Coulomb repulsion is accelerated as the liquid drops become smaller, the generation of the target ions is also accelerated.
In order to improve the sensitivity of analysis by using an LC-MS thus structured, it is important to ionize the liquid sample efficiently at the interface part 20 and to introduce the generated ions efficiently into the quadrupole filter 37 (or any other kind of mass analyzer). These can be accomplished only if various parameters for the operations of the interface part 20 and the MS part 30 (such as the temperatures and applied voltages) are properly set. With a prior art LC-MS, the voltages to be applied to the solvent-removing tube 23 and the deflector electrodes 34 are adjusted such that the number of ions reaching the detector 38 will be maximized, for example, when a standard sample containing a specified component is introduced, that is, such that the peak of the mass spectrum corresponding to this specified component will reach a highest value. In practice, however, the voltage at which the solvent-removing tube 23 and the deflector electrodes 34 pass the ions most efficiently depends on the mass number of these ions. When a measurement is taken by scanning over a certain range of masses, therefore, the solvent-removing tube 23 and the deflector electrodes 34 are not necessarily in optimum conditions for passing the ions, and this has been one of the factors preventing the prior art LC-MS from operating under an optimum condition in terms of the sensitivity and accuracy of the detection.
It is therefore an object of this invention in view of these problems to provide an improved mass spectroscope for a liquid chromatograph capable of efficiently introducing target ions to be analyzed into the mass spectroscope part such that its detection sensitivity and detection accuracy can be improved.
A mass spectroscope for a liquid chromatograph embodying this invention, with which the above and other objects can be accomplished, may be characterized not only as comprising an interface including an atomization chamber into which a liquid sample from the liquid chromatograph is sprayed to be converted into ions, an intermediate chamber at a reduced inner pressure and a detection chamber at a lower inner pressure than the intermediate chamber and containing a mass analyzer but also as having a solvent-removing tube for causing liquid droplets containing these ions to pass through from the atomization chamber into the intermediate chamber, means for causing these ions to travel along a travel path through the intermediate chamber into the detection chamber, a deflector having at least one pair of planar electrodes disposed inside the intermediate chamber and opposite each other sandwiching the travel path in between, a voltage generating means for applying a variable DC voltage to the solvent-removing tube, separate voltage generating means for independently applying a different variable DC voltages to each of these electrodes, a memory which stores data on voltages to be applied to the solvent-removing tube and to the electrodes for optimizing efficiency with which ions with different mass numbers are received by the mass analyzer, and a control unit for applying a specified voltage to the mass analyzer and simultaneously controlling the voltage generating means so as to have voltages selected according to the data stored in the memory and applied to the solvent-removing tube and to the electrodes.
In using a mass spectrometer according to this invention, one or more standard samples containing components with different mass numbers are preliminarily analyzed to determine optimum voltages to be applied to the solvent-removing tube and the deflector electrodes for each of the mass numbers. A voltage scan pattern is produced on the basis of these data such that optimum or nearly optimum voltages can be applied corresponding to all mass numbers of interest, and the pattern thus produced is stored in a memory device. At the time of a measurement, the voltages to be applied are varied such that only the ions having particular mass numbers are sequentially allowed to pass through. At the same time, the control unit controls the voltages to be applied according to the pattern stored in the memory such that each group of ions having a particular mass number can pass through the solvent-removing tube and the deflector electrodes under an optimum or nearly optimum condition to be received by the mass analyzer.
Since preferred pattern shapes are empirically known, it is preferable to produce such patterns according to an algorithm, based on separate data which may be obtained by analyzing a plurality of standard samples, as explained above.